In electrophotography, for example, an electrostatic latent image is principally formed on a latent image carrying member made of a photoconductive photoreceptor by means of an electrostatic charge or light exposure; then, the generated image is developed with a toner. The toner image obtained is, after being transferred on a support such as an image transfer sheet, fixed on the material with a means that includes heat or pressure to produce a visible image.
The production of such a visible image by way of the electrostatic image should preferably be executed at a high speed. In view of this, the heat roller fixing method, which features better efficiency and is more beneficial than other methods, has widely been practiced.
However, demand for more speed is mounting recently, and high speed fixation of the toner image is one of the essential prerequisites for satisfying this requirement.
The heat roller fixing method requires an image developing toner with an excellent low temperature fixing property so that the toner image may be rapidly fixed. For this purpose, a resin contained in the toner as a binder should have a lower softening point. However, if a toner binder has a lower softening point, a so-called offset phenomenon tends to occur. In this phenomenon, the toner, in order to form an image at fixation sequence partly transfers to the heat roller; then, the transferred toner is again transferred to the image transfer sheet that follows, contaminating the image.
For this reason, a toner which comprises, as a binder, a non-linear polyester obtained from the polymerization of monomer composition comprising an etherified bisphenol monomer, dicarboxylic acid monomer, a polyhydric alcohol containing at least 3 hydroxy groups and/or a polycarboxylic acid monomer containing at least 3 carboxylic acid groups was proposed. (See Japanese Patent Publication Open to Public Inspection No. 37353/1982 and No. 208559/1982; hereinafter referred to as Japanese Patent O.P.I. Publication.)
With the prior art above, the toner was provided with an offset prevention feature by incorporating, as a binder, a polyester obtained by inter-linking a linear polyester comprising an etherified bisphenol and a dicarboxylic acid monomer with monomer components including a polyhydric alcohol monomer containing at least 3 hydroxy groups and/or a polycarboxylic acid monomer containing at least 3 carboxylic acid groups.
This type of toner, however, has a relatively high softening point; therefore, it cannot satisfactorily perform fixation at low temperatures, and, it is difficult to achieve sufficiently rapid fixation with this type of toner.
In order to eliminate such a drawback, a certain toner having the following characteristics has been proposed. (See Japanese Patent O.P.I. publication No. 57-109825/1982, Japanese Patent Application No. 109539/1984, Japanese Patent O.P.I. Publication No. 7960/1984.) A polymer contained as a binder in the toner, having a saturated or unsaturated aliphatic hydrocarbon group with 3-22 carbon atoms in a side chain, is a non-linear copolymer obtained from the polymerization of monomer composition comprising an etherified bisphenol monomer, a dicarboxylic acid monomer, a polyhydric alcohol monomer containing at least 3 hydroxy groups and/or polycarboxylic acid monomer containing at least 3 carboxylic acid groups.
With such a toner, a sufficient fixing property is possible at low or medium speed fixation, however, if high speed fixation is continuously exercised at a fixing rate in excess of 50 A-3 size sheets/min. for example, because the papers are fed to the heat roller at extremely short intervals, the papers absorb a great amount of heat from the heat roller, causing the temperature of the roller to drop drastically, often resulting in improper fixation.
In view of such a drawback, a measure may be incorporated to improve the property of low temperature fixing by decreasing the molar weight of a binder which decreases toner viscosity during the fixation. However, such a measure simultaneously accompanies the deterioration of anti-offset properties of the toner; in addition, as the toner tends to aggregate, the movement of the toner as unit particles is hindered, making satisfactory image development impossible.